Redox Titration - University of Massachusetts Boston

1 Properties of Umass Boston Redox Titration Properties of Umass Boston Redox Titration Redox Titration is based on the Redox reaction (oxidation-reduction) between analyte and titrant. Ce4+ + Fe2+ Ce3+ + Fe3+ Properties of Umass Boston Position of the end point Properties of Umass Boston Determine the end point Indicator electrode Redox indicators the indicator has different color at reduction and oxidation state. Non Redox indicator change color when excess amount of titrant exists, Starch changes to deep blue color when excess amount I2 remains Properties of Umass Boston Redox indicator Properties of Umass Boston Redox indicators The indicator potential transition range should overlap the steep part of the Titration curve Properties of Umass Boston Adjustment of oxidation state Sometimes the oxidation states of analytes need to be adjusted before Titration oxidants need to be removed.

2 Pre-oxidation: S2O8 2- (peroxydisulfate, persulfate): S2O82- + 2e 2SO42- S2O82- +2H2O 4SO42- +O2 + 4H+ (boiling) sometime, Ag+ is needed as catalyst S2O82- + Ag+ SO42- + SO4- + 4Ag2+ H2O2 Properties of Umass Boston Adjustment of oxidation state Pre-reduction SnCl2, etc Reductor: Jones reductor: Zn-Zn amalgam Walden reductor: Ag/AgCl Properties of Umass Boston Example of typical oxidants Potassium Permanganate (KMnO4) MnO4- + 8H++5e Mn2++4H2O Eo= (pH<1) MnO4- + 4H++3e MnO2 (s)+2H2O Eo= (pH 7) In alkaline solution: MnO4- + e MnO42- Preparation, standardization and storage Not a primary standard, MnO2 impurity To prepare, dissolve KMnO4 in DI water, boil in a hour to make sure oxidize all organics, filter through sintered glass filter (why?)

3 , store in dark glass. Use either Fe2+ or sodium oxalate (Na2C2O4) for standardization Properties of Umass Boston Properties of Umass Boston Methods involving Iodine Iodine is hardly soluble in water, but very soluble in Iodide solution I2 + I- I3- (triiodide) Iodimetry: I3- as titrant Iodometry: I3- is produced by adding oxidating analytes into excess amount of Iodide (I-) Indicator starch, added near the end point (Iodometry), but at the beginning for Iodimetry. Standardization (pure enough for primary standard, but evaporates during weighting) Weight and dissolve in I- Use Arsenious Oxide (As4O6) or Soduim Thiosulfate (Na2S2O4)

4 For standardization Storage, no light, no oxygen Properties of Umass Boston Properties of Umass Boston Properties of Umass Boston Can be used to analyze oxidants and reductants Properties of Umass Boston Electroanalysis Properties of Umass Boston Thermodynamic and Kinetic If there is no net current passing through the electrochemical cell, the system is at thermodynamically equilibrium state. The potential can be calculated by Nernst equation. If there is net current passing through, the system is away from the thermodynamic equilibrium state, thus the potential can not be calculated by Nernst equation Properties of Umass Boston Kinetics of electrochemical system When Current passing through the system.

5 The potential will move away from that of equilibrium state for three reasons Overpotential IR drop Concentration polarization Properties of Umass Boston Overpotential Overpotential is to overcome the activation of energy barrier of the reaction Properties of Umass Boston IR drop Inevitably, the electrochemical system will have ohimic resistance, when the current runs through the system, the voltage will drop due the ohmic resistance: V= IR Properties of Umass Boston Concentration Overpotential Due to the concentration gradient from the surface of the electrode to the bulk of the solution.

6 Properties of Umass Boston Three electrode system Working electrode: where the analytical reaction happens Counter electrode: for the current to flow and make a close circuit. Reference electrode: no current pass through and serve as a reference point for potential measurement Why three electrode system is needed?? Properties of Umass Boston Amperometry Diffusion limited current Properties of Umass Boston Three electrode system Working electrode: where the analytical reaction happens Counter electrode: for the current to flow and make a close circuit.

7 Reference electrode: no current pass through and serve as a reference point for potential measurement Why three electrode system is needed?? Properties of Umass Boston Polarography: Historical Significance Polarograph developed by Jaroslav Heyrovsky in 1922. Heyrovsky won Nobel Prize Polarograph is THE most widely used electro analytical method Still one of the most reliable analytical method for ppm level impurities. Properties of Umass Boston Setup Polarograph Dropping Hg electrode (DME) Working under limited diffusion current Properties of Umass Boston Polarography curve Properties of Umass Boston Example Cd2++2e Cd E (vs SCE)= V Zn2++2e Zn E (vs SCE)= V Properties of Umass Boston Electrogravimetric Analysis Quantitatively electrochemically deposit analyte onto electrode, the amount of analyte can be measured by measure the weight change of the electrode.

8 The end point determination Color change Cu2+ + 2e Cu Deposition on fresh electrode surface. Properties of Umass Boston Electrogravimetric Analysis Current-Voltage relationship Interference of other ions To limit the potential, depolarizer is added to prevent hydrogen or oxygen evolution. NO3- + 10H++ 8e NH4++3H2O Prevent hydrogen evolution Properties of Umass Boston Coulometry Generating titrate electrochemically, the amount of titrate generated can be measured by counting electrons. Detector anode 2Br- Br2 + 2e Detector cathode Br2 + 2e Br- Properties of Umass Boston Coulometry Advantage of Coulometry Titration Precision Sensitivity Generating otherwise unstable titrant in-site Constant current Q=I x t Constant potential 3-electrode system Q= I x t More sensitive and selective Properties of Umass Boston Cyclic voltammetry Linear Sweep Voltammetry (LSV) Cyclic Voltammetry (CV)-good for the study of reaction mechanism Initial potential End potential Sweep rate (mV/s)

9 Properties of Umass Boston karl fischer Titration of H2O Detecting trace amount of water in organic solution. An example of coulometric Titration